The proposed research program seeks to detail the structural features of complexes of carcinogenic polycyclic hydrocarbons with nucleic acids and to elucidate the intermolecular interactions which stabilize them. Our approach will be through the utilization of magnetic properties in the photoexcited triplet states of the carcinogen to probe the orientations and interactions within the complex, using optically-detected magnetic resonance spectroscopy. Our experiments will focus on complexes of polycyclic hydrocarbons bound covalently to DNA, the structures of which may be of importance in formulating mechanisms of chemical carcinogenesis at the molecular level.